Unarmored Threespine Stickleback - Gasterosteus aculeatus williamsoni

The unarmored threespine stickleback a subspecies of the threespine stickleback is found only in North America and its recognised range is only in parts of California. Other threespine sticklebacks can be found circum polar. Mostly fresh water only but some are anadromous. That is they live in saltwater and return to freshwater to breed.

There is a tremendous amount of genetic variation between the three subspecies of threespine stickleback. In cases where two of the subspecies occupy the same waters they do not interbreed although they are genetically similiar enough to do so.

Only the males care for the eggs once they are fertilised. Parental care is intense, involving nest maintenance and fanning of the eggs to ensure a fresh water supply, even at night. Males build the nests from vegetation, sand, pebbles and other debris, adhering the material together with spiggin, a proteinaceous glue-like substance secreted from the kidneys.

Three-spined sticklebacks have recently become a major research organism for evolutionary biologists trying to understand the genetic changes involved in adapting to new environments. Other species pairs which consist of a well-armored marine form and a smaller, unarmored fresh water form are being studied in ponds and lakes in Southcentral Alaska that were once marine habitats such as those uplifted during the 1964 Alaskan Earthquake. The evolutionary dynamics of these species pairs are providing a model for the processes of speciation which has taken place in less than 20 years in at least one lake. In 1982, a chemical eradication program at Loberg Lake, Alaska, killed the resident freshwater populations of sticklebacks. Oceanic sticklebacks were introduced and colonized the lake. In just 12 years beginning in 1990, the frequency of the oceanic form dropped steadily, from 100% to 11%, while a variety with fewer plates increased to 75% of the population, with various intermediate forms making up another small fraction.

This rapid evolution is thought to be possible through genetic variations that confer competitive advantages for survival in freshwater when conditions shift rapidly from salt to freshwater. However, the actual molecular basis of this evolution still remains unknown.